Researchers from the University of Texas recruited six young, healthy, lean people to take part in their study, which compared the formation of fat (lipogenesis) following consumption of pure glucose, or combinations of glucose and fructose.
The results are published in the Journal of Nutrition.
“Our study shows for the first time the surprising speed with which humans make body fat from fructose,” said Elizabeth Parks from the UT’s Southwestern Medical Center.
“[Fructose, glucose and sucrose] can be made into triglycerides, a form of body fat; however, once you start the process of fat synthesis from fructose, it’s hard to slow it down,” she added.
However, Dr Parks was quick to point out that it is misleading to suggest the consumption of a specific food or food ingredient was the cause of obesity and the rise of type-2 diabetes.
“There are lots of people out there who want to demonize fructose as the cause of the obesity epidemic,” she said. “I think it may be a contributor, but it’s not the only problem. Americans are eating too many calories for their activity level. We’re overeating fat, we’re overeating protein; and we’re over-eating all sugars,” she said.
Parks and co-workers recruited four men and two women with an average age of 28, and an average BMI of 24.3 kg per sq. m. The volunteers consumed three soft drinks on three separate occasions; one was sweetened with 100 per cent glucose, the second with 50 per cent glucose and 50 per cent fructose, and the third with 25 per cent glucose and 75 per cent fructose. The tests were random and blinded.
The UT researchers report that lipogenesis increased from 7.8 per cent for the glucose beverage, to 15.9 per cent after the 50:50 beverage and 16.9 per cent after the 25:75 beverage.
Moreover, blood triglyceride levels were between 11 and 29 per cent higher after consumption of the 50:50 and 25:75 beverage, compared to the 100 per cent glucose drink.
“The message from this study is powerful because body fat synthesis was measured immediately after the sweet drinks were consumed,” said Dr. Parks said. “The carbohydrates came into the body as sugars, the liver took the molecules apart like tinker toys, and put them back together to build fats. All this happened within four hours after the fructose drink. As a result, when the next meal was eaten, the lunch fat was more likely to be stored than burned.
“This is an underestimate of the effect of fructose because these individuals consumed the drinks while fasting and because the subjects were healthy, lean and could presumably process the fructose pretty quickly. Fat synthesis from sugars may be worse in people who are overweight or obese because this process may be already revved up,” she added.
Metabolism of fructose
The metabolism of glucose and fructose is different, said the researchers. In humans, triglycerides are predominantly formed in the liver. This organ decides what to do when it encounters glucose. The sugar is either stored as glycogen, burnt for energy or turn the glucose into triglycerides.
On the other hand, fructose bypassing the liver and floods the metabolic pathway, said the researchers.
"It’s a less-controlled movement of fructose through these pathways that causes it to contribute to greater triglyceride synthesis," said Parks. "The bottom line of this study is that fructose very quickly gets made into fat in the body.”
The work was supported by the National Institutes of Health, the Cargill Higher Education Fund and the Sugar Association.
Research from the University of Florida in 2005 reported similar results. In that instance the research implicated a rise in uric acid in the bloodstream that occurs after fructose is consumed. That temporary spike blocks the action of insulin, which typically regulates how body cells use and store sugar and other food nutrients for energy.
If uric acid levels are frequently elevated, over time features of metabolic syndrome may develop, including high blood pressure, obesity and elevated blood cholesterol levels.
"We cannot definitively state that fructose is driving the obesity epidemic," said researcher Richard Johnson, from UF's College of Medicine Johnson, in 2005. "But we can say that there is evidence supporting the possibility that it could have a contributory role - if not a major role."
Source: Journal of NutritionJune 2008, Volume 138, Pages 1039-1046 "Dietary Sugars Stimulate Fatty Acid Synthesis in Adults"Authors: E.J. Parks, L.E. Skokan, M.T. Timlin, C.S. Dingfelder